The present invention relates to a radial carcass tire having a crown reinforcement of small hysteresis loss, with improved rolling resistance.
Since savings in fuel and the need to protect the environment have become a priority, it is desirable to employ rubber compositions which can be used for the manufacture of various semifinished products entering into the construction of tires, such as, for instance, cushion and calendering rubbers for cord fabric plies or treads in order to obtain tires having reduced resistance to rolling.
It is known to the person skilled in the art that the first and main factor affecting the resistance to rolling of a radial carcass tire resides in the rubber composition forming the tread of the tire.
Significant progress has been achieved in the field of tires having at the same time reduced resistance to rolling, excellent adherence both on dry and on snowy ground, very good resistance to wear, and reduced rolling noise, by the use, as tire tread, of a rubber composition, described in European Patent Application EP 0 501 227, which is vulcanizable by sulfur, obtained by the thermo-mechanical working of a conjugated diene polymer and an aromatic vinyl compound prepared by solution polymerization with 30 to 150 parts by weight of a highly dispersible precipitated silica to 100 parts by weight of elastomer.
It is also known to the person skilled in the art that the second factor which exerts a dominant influence with respect to the rolling resistance of a radial carcass tire is the crown reinforcement.
The crown reinforcement is generally formed of two plies of cord fabric having non-stretchable cords parallel to each other in one ply and crossed from one ply to the next, forming equal or different angles of between 10.degree. and 45.degree. with the circumferential direction. The cords are either metal cables, in particular of steel, or synthetic textile cords, in particular of aramids. These plies which are inclined with respect to the circumferential direction are referred to as "working" plies.
Particularly in the case of tires intended for high speed passenger vehicles the crown reinforcement can have, in addition to the working plies, one or more cord fabric plies, strips of cord fabric or of helically wound thread, the component cords or threads of which are substantially not inclined with respect to the circumferential direction, that is to say, they form an angle of zero degrees or close to zero degrees and are known as "zero-degree plies". The cord is generally a cord of synthetic textile, in particular a polyamide.
The calendering rubber used for the working plies is generally formed exclusively of natural rubber or by a blend of natural rubber and a diene synthetic rubber or a mixture of diene synthetic rubbers, natural rubber being present, however, in a preponderant amount by weight. This calendering rubber is ordinarily reinforced by carbon black as a major filler. However, it is known to the person skilled in the art, for instance from French patent application FR 80 22131 and U.S. Pat. No. 4,229,640, to use silica in a small amount, on the order of 10 to 15 parts by weight, in order to increase the adherence of the rubber to the metal cable and in particular to a brass-plated metal cable. However, the silica is used in combination with a reinforcing resin, generally one having a base of resorcinol, in order to increase the modulus of the calendering rubber, it being well-known to the person skilled in the art that the use of silica in tires as reinforcing filler has been retarded for a long time due, inter alia, to the lower modulus of elasticity of silica-filled rubber mixes.
The use of silica in tires also been extensively retarded due to difficulties in working resulting from silica-silica interactions which tend, in raw state, to cause an agglomerating of the silica particles before and even after mixing, making the working more difficult than with carbon black and leading to hard raw rubbers as soon as the percentage of silica used in the composition is relatively high. Due to their hardness, such rubbers are unsuitable as calendering rubbers for cord fabric and furthermore give rise to major problems of coherence in cured state. The use of such rubbers leads to a premature separation of the cords from the rubber, in particular at the ends of the working plies.
U.S. Pat. No. 5,066,721 describes a rubber composition having a base of a diene polymer functionalized by means of a special silane compound having a non-hydrolyzable aryloxy group capable of being used as calendering rubber for the cord fabric, in particular of crown reinforcement working plies and capable of containing up to 20 parts by weight of a conventional silica, that is to say a silica which has a high CTAB specific surface area of more than 100 m.sup.2 /g and is only slightly dispersible. The improvement in the rolling resistance of the tire is essentially due to the nature of the functionalized diene polymer, which makes it possible to increase in a very small proportion the silica content of the calendering rubber, but also, at the same time, to increase the problem of internal coherence of the rubber and the risk of premature separation of the cord fabric from the rubber.